TCP Enhancement for Random Loss

Jiang WuComputer Science

Lakehead University

Content TCP Issue Queueing Model Model Analysis Enhancement Proposal Conclusion

TCP Issue TCP ref[1], [3]

Congestion control algorithm Reduce transmission rate whenever packet loss

occurs Issue ref[4]

Don’t distinguish between congestion and random loss

Take incorrect action to random losses Suffer performance degradation in wireless

network

Queueing Model ref[5]

TCP Sender

Router Buffer

Queueing System

TCP Receive

r

Router

Some parameter of the queueing system: Queueing delay: the time a packet takes waiting in the queue Transmission delay: the time the router takes to put a packet to the

transmission channel Suppose the size of the packets is constant, the router can be modeled

as an M/D/1 queue because: Packets arrive according to Poisson process Transmission delay is constant 1 server (Router) for the queue

M/D/1 Queue Properties ref[5]

M/D/1 queue properties: Let the packet arrival rate to be , transmission delay to be , then the load of the link is

defined as

The expectation of the queueing delay

The expectation of queue length Q is

)1(2)(

WE

W

)1(2)(

2

QE

M/D/1 Queue Characteristics

0

2

4

6

8

10

E( W) / E( Q)

E( W) / 0 0. 06 0. 13 0. 21 0. 33 0. 5 0. 75 1. 17 2 4. 5 9. 5

E( Q) 0 0. 01 0. 03 0. 06 0. 13 0. 25 0. 45 0. 82 1. 6 4. 05 9. 02

0 0. 1 0. 2 0. 3 0. 4 0. 5 0. 6 0. 7 0. 8 0. 9 0. 95

Calculate Queue Length Round Trip Time (RTT): The time period between a packet

is sent by the sender and its ACK is received by the sender. It consists of:

Queueing Delay, a random variable >= 0 Transmission delay Propagation delay: the time the packet travels on the line

ACK delay: the time the ACK travels from the receiver to the sender RTTmin : The minimum RTT observed by the sender Calculate the load

)1(2

)(min

WERTTRTT

min

min

22

22

RTTRTT

RTTRTT

( is unknown yet)

Get Transmission Delay

Packet pairs in TCP ref[1], [3]

Get transmission delay ref[1]

A by-product: maximum bandwidth of the bottle link

Calculate the parameters:

Router RouterSender Receiver

maxBW

/max packetSizeBW

min

min

22

22

RTTRTT

RTTRTT(All parameter is known now)

Detect the Congestion

Throughput: new packets received by receiver in unit time The knee occurs when the average queue length is one ref[2]

732.0 366.1/)( WE1)( QE

Ref[2]

Enhancement Proposal Enhancement to TCP

Detect the minimum RTT When there are 2 packets ready to be sent, send them

back-to-back, and measure the space between the ACK Calculate the load When a packet loss occurs when , don’t

lower the transmission rate

Anticipated result TCP performance will not degrade heavily due to

transmission loss

732.0

Research Plan Design the enhancement algorithm Implement the enhancement Evaluate the performance

improvement One paper before March 2004 Compare the performance to others One paper

Reference [1] V. Jacobson. Congestion avoidance and control. In

ACM SIGCOMM '88, 1988. [2] K. Ramakrishnan and R. Jain, “Congestion avoidance

in computer networks with a connectionless network layer” Tech. Rep., DEC, August 1987.

[3] RFC2581 TCP Congestion Control [4] Pan, J.; Mark, J.W.; Shen, X. TCP performance and its

improvement over wireless links. Globecom 2000. IEEE [5] Robert B. Cooper. Introduction To Queueing Theory.

Second Edition. 1981. North Holland, New York Oxford